The present invention relates to a pressure sensor and, more particularly, to a pressure sensor which can compensate for a temperature variation.
A pressure sensor is proposed, which develops a signal of which frequency varies in response to variations of a detected pressure. Such a digital pressure sensor is applicable to a sphygmomanometer including a digital display. A typical digital pressure sensor of the above-mentioned type is described in my copending application Ser. No. 946,089 "DIGITAL PRESSURE SENSOR" filed on Sept. 26, 1978. In such a digital pressure sensor, there is a possibility that the output frequency varies, without regard to the variation of the detected pressure, when a temperature varies. This will preclude an accurate pressure detection.
Accordingly, an object of the present invention is to provide a pressure sensor which can compensate for a temperature variation.
Another object of the present invention is to provide a pressure sensor suited for a sphygmomanometer including a digital display.
Still another object of the present invention is to provide a pressure sensor of a simple construction, and which ensures an accurate pressure detection.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
To achieve the above objects, pursuant to an embodiment of the present invention, a bellows responsive to a pressure to be detected and a frequency variation means for varying an oscillation frequency of an oscillator in response to variations of the detected pressure are disposed on a supporting table. The supporting table is so constructed as to show the thermal expansion which corresponds to the total thermal expansion of the bellows and the frequency variation means. More specifically, the total thermal expansion, due to temperature variations, of the bellows and the frequency variation means in a predetermined direction is identical with that of the supporting table in the same direction.
In a preferred form, the supporting table is made of aluminum and shows the thermal expansion of 2.7.times.10.sup.-3 mm when the temperature increases by 1.degree. C. The bellows is made of phosphor bronze and shows the thermal expansion of 4.times.10.sup.-4 mm when the temperature increases by 1.degree. C. The frequency variation means comprises a core attached to the bellows and a winding wound around a bobbin which is secured around the core. The core is made of ferrite and shows the thermal expansion of 2.times.10.sup.-4 mm when the temperature increases by 1.degree. C., and the bobbin is made of a phenol resin material and shows the thermal expansion of 2.1.times.10.sup.-3 mm when the temperature increases by 1.degree. C.